The primary aims of this grant are to: (1) evaluate the stability of individual differences in infants'behavior (positive and negative engagement, self-regulatory behaviors), vagal tone and cortisol response assessed during a social stress, maternal Face-to-Face Still-Face Paradigm over a 3- or a 6-month time interval;(2) evaluate infants'memory for the still-face over the 3- or 6-month time interval;and (3) evaluate the relation between infant reactivity and memory. Ten independent groups of mother-infant dyads (N = 340, n = 34 per group) balanced for infant gender will comprise the sample. Six groups will be randomly assigned to an experimental condition. For infants in the experimental condition, the 1st exposure to the FFSF will take place when the infants are 6, 9, or 12 months of age, and their respective 2nd exposure will be either 3 or 6 months later, at 9, 12, 15 or 18 months of age. Infants in the control condition will be videotaped once in the FFSF, at the age corresponding to the 2nd exposure for infants in the experimental conditions (at 9, 12, 15, or 18 months of age). For all groups measures of infant gaze during the SF will be coded as a measure of memory. Infants'vagal tone (VT), cortisol response, and negative engagement states and self-regulatory behaviors during the FFSF paradigm will be scored microanalytically from videotapes will be collected at each visit as measures of reactivity. Variations in infants'cardio-respiratory activity (vagal tone) will be assessed using the method developed by Porges, and cortisol response will be measured using salivary cortisol with methods developed by Gunnar. A measure of infant perceived temperament will be derived at each visit from temperament questionnaires completed by the mother: the Infant Behavior Questionnaire (IBQ) for infants at 6, 9, or 12 months and the Toddler Assessment of Behavior Questionnaire (TABQ) for infants at 15 or 18 months of age. This study will provide valuable longitudinal on a standard and widely used stress paradigm and make a significant contribution to our understanding the stability of individual differences in infants'behavior, affect, physiological regulation, and memory during social stress.